SEED PLANTS 163 
of the size of a large plum. The very large macrospore 
(Fig. 40, B, ma) has a definite thick membrane like that of 
the ferns, but is retained permanently within the sporan- 
gium. So far as it is known, the development of the 
Fic. 40 (Cycadacew).— A, a sporophyll of Cycas circinalis, with six 
ovules (macrosporangia), ma; B, longitudinal section of a young ovule 
of C. revoluta, showing the single large macrospore, ma; C, asporophyll 
from the male cone of C. revoluta, showing the lower surface covered 
with groups or sori of microsporangia, mi; D, a single sorus of five 
microsporangia; E, a microspore (pollen-spore), showing the rudimen- 
tary. antheridium, an; the larger antheridial cell later gives rise to 
two large spermatozoids; F,a young leaf of C. revoluta, showing the 
fern-like coiling of the divisions; G,a scale from the female cone of 
Zamia integrifolia, with two ovules, ma; H, section through the ovule 
at the time of fertilization; pc, the pollen-chamber with three germi- 
nating pollen-spores; g, the vegetative tissue of the female gametophyte 
contained within the macrospore; ar, two archegonia; I, a spermato- 
zoid of Zamia, showing the numerous cilia, c. (Figs. H, I, after Webber.) 
gametophyte is much like that of Isoétes or Selaginella, 
but the details are still somewhat imperfectly known. 
The gametophyte, if fertilization is not effected, may 
grow out beyond the spore and develop chlorophyll, 
